Q: What degradation mechanism can result from interaction of molten core and concrete?
A: If molten core contacts concrete, a chemical reaction will occur that generates noncondensable gases. This is a slow degradation process relative to many of the other failure
mechanisms in play (e.g., hydrogen detonation). Flooding of the drywell will reduce the molten core – concrete interaction.
Q: What degradation mechanism exists for concrete under elevated drywell temperature?
Concrete will degrade when exposed to high temperature. At 1400 – 1500 °C, concrete will lose its structural integrity. These temperatures could be reached if concrete is in direct contact with
molten core. If there is direct contact, the temperature of the concrete can raise enough, in the localized area, to cause failure. Concrete in other areas, can weaken, but not lose its integrity.
For this reactor design, the concrete of concern is the basemat, which does not have a structural function. More detailed information on this degradation mechanism is available in NUREG/CR-7031, “A Compilation of Elevated Temperature Concrete material Property Data and Information for Use in Assessments of Nuclear Power Plant Reinforced Concrete Structures,” December 2010.
Neither of these degradation mechanisms, molten core – concrete interaction or prolonged exposure to elevated drywell temperature, will cause a sudden catastrophic release of radiation that would necessitate evacuation of personnel. The only caveat to this is the indirect overpressurization of containment due to non-condensable off gassing from the molten core – concrete interaction. Again, this is a slow process, through pressurization, but it could potentially lead to a quick change in containment integrity at the time of failure.